Buchanan County is a county located in the U.S. state of Missouri. As of the 2010 United States Census, the population was 89,201. Its county seat is St. Joseph. When originally formed in 1838, the county was named Roberts County, after settler Hiram Roberts. It was renamed in 1839 for James Buchanan, then a U.S. Senator and later President of the United States. The county was formed from land annexed to Missouri, as were five other counties.
According to the U.S. Census Bureau, the county has a total area of 415 square miles (1,070 km), of which 408 square miles (1,060 km) is land and 6.6 square miles (17 km) (1.6%) is water.
As of the census of 2000, there were 85,998 people, 33,557 households, and 21,912 families residing in the county. The population density was 210 people per square mile (81/km²). There were 36,574 housing units at an average density of 89 per square mile (34/km²). The racial makeup of the county was 92.73% White, 4.36% Black or African American, 0.42% Native American, 0.45% Asian, 0.02% Pacific Islander, 0.65% from other races, and 1.37% from two or more races. Approximately 2.43% of the population were Hispanic or Latino of any race.
There were 33,557 households out of which 30.60% had children under the age of 18 living with them, 49.30% were married couples living together, 12.00% had a female householder with no husband present, and 34.70% were non-families. 28.90% of all households were made up of individuals and 12.50% had someone living alone who was 65 years of age or older. The average household size was 2.42 and the average family size was 2.98.
In the county, the population was spread out with 24.30% under the age of 18, 11.00% from 18 to 24, 28.50% from 25 to 44, 21.20% from 45 to 64, and 15.00% who were 65 years of age or older. The median age was 36 years. For every 100 females, there were 96.70 males. For every 100 females age 18 and over, there were 93.90 males.
The median income for a household in the county was $34,704, and the median income for a family was $42,408. Males had a median income of $31,697 versus $21,827 for females. The per capita income for the county was $17,882. About 8.50% of families and 12.20% of the population were below the poverty line, including 15.00% of those under age 18 and 9.60% of those age 65 or over.